Nimrod was a slave trader, the son of Cush. There is no mention that he was offspring of the “sons of God”.
But why would God make the mistake of letting the bloodline of the people He intended to eradicate or imprison slip through?
I have no earthly idea. But there are those theologians who hold to the idea that the Nephilim bloodline existed past the Flood and there are those who do not.
They both make very good points.
As an aside, Nimrod was a good bit more than a slave trader. The word used to describe Nimrod in Gen 10 as a mighty man was the same used to describe the Nephilim in Gen 6 (gibbowr). The only usage of that word in Genesis is in Gen 6 and Gen 10, to describe the Nephilim (used once) and to describe Nimrod (used 3 times).